Lighting

Lighting impacts the street environment in two major ways: the performance and quality of the lighting affects safety and navigability, and the street light equipment (usually a series of poles and luminaires) can help shape the character and aesthetics of the streetscape and neighborhood.

Street Lighting Basics: The appropriate design and placement of street lighting is determined according to several interrelated criteria including street use, street type, the degree of pedestrian use, the presence of street trees, the intended illumination levels for drivers and pedestrians, the need for lighting uniformity, energy efficiency considerations, dark sky ordinances, and the design character of the street.

These criteria will affect decisions about the height of the light source above grade, the horizontal spacing of pole/luminaire combinations, luminaire type, lamp type, and the architecture of the pole/luminaire. For most roadways, an electrical engineer or lighting designer recommends particular treatments and design elements, and an urban designer, landscape architect or architect may also be involved for streets with a more specific or intricate intended design character. The Illuminating Engineering Society of North America (IESNA) establishes national standards for the engineering and performance of roadway lighting.

Pole and luminaire configurations: Street lighting poles and luminaires can be divided into two main categories based on size and purpose: roadway height (16’ to 40’) and pedestrian height (10’ to 16’ feet). Luminaire mounting configurations are referred to as post-top if the fixture is mounted atop a pole, or pendant mount if the fixture is mounted on an arm extending from the pole, either suspended from above or from the side.

Lighting control and distribution: To the degree possible, lighting designers generally strive to create a uniform level of light throughout a street and/or sidewalk environment. These designers will typically begin by setting the height, spacing, and orientation of lamps whose light is modified and controlled by the design of the luminaires.

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Brightness: In the past, lighting for most streets was designed to a 0.1 footcandle “maintained” minimum standard; however, many retail areas and pedestrian-oriented streets are now designed to a 1.0 footcandle maintained level, with some cities such as Phoenix, AZ requiring 5.0 footcandles maintained in downtown. As urban areas develop, glare and light pollution are becoming increasingly problematic.

Glare, an uncomfortably strong light source, is both a nuisance and a safety hazard for drivers and pedestrians, while light pollution of the night sky is typically considered an aesthetic and efficiency issue. Cut-off lighting, a treatment designed to block any light emitted above the horizontal plane of the lamp (shown at right), can be used to minimize light pollution and shield upper floor windows of buildings, and will soon be required in most states.

Reflectors, fixtures that control light using mirrored surfaces within a fixture, and refractors, which use lenses (typically Fresnel lenses, like those of a lighthouse) to control and direct light, are also used to control and minimize light pollution.

The globe of a light fixture (the outer transparent or translucent shell around a lamp) is referred to as refractive, when it contains Fresnel lens elements. Diffuser, globes are translucent and distribute the lamp’s brightness over a larger surface area without directional control, thus reducing glare but not necessarily light pollution. Using a combination of the lighting control methods described above within a fixture can allow for greater spacing between luminaires and minimize installation and operating costs.

Lamp types, energy efficiency, and lighting color: No current lamp technology is perfect in terms of energy efficiency, longevity, and color quality. The most energy-efficient lamp technologies commercially available for street lighting at present include high-intensity discharge (HID) lamps, mercury vapor, and fluorescent lamps. The four types of high-intensity discharge lamps - high-pressure sodium, low-pressure sodium, metal halide, and induction - are all efficient in “lumens per watt” and relatively long-lived.

Light-emitting diodes, or LED lighting, is considered moderately efficient, but is not yet a cost-effective option, nor is it widely distributed for commercial use; however LED use and affordability are expected to increase over the next few years. Mercury vapor and fluorescent lamps are less commonly used for street lighting today due to slightly lower efficiency, a shorter lamp life, power/size limitations, and other technical issues. Both standard and halogen incandescent lamps are considered non-efficient lighting sources due to their high energy consumption and short lamp life. Incandescent lamps are rarely used for area illumination, but can be useful for accent lighting.

Lamps also have two important color characteristics: the color of the emitted light (measured in degrees Kelvin/color temperature) and the accuracy of colors as seen under the light (measured as a Color Rendering Index – CRI).

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High-pressure sodium measures at about 2200K (peach-colored orange, as shown in the image below), a household incandescent lamp rates 2700K (considered the most desirable color range), and a bright white metal halide lamp used in stadium lighting might be 4000K.

The most commonly used lamp type for street lighting in North America is the high-pressure sodium lamp, due to its high energy efficiency in terms of “lumens per watt” and its relatively long lamp life (20,000+ hours).

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The peach-orange light emitted by high-pressure sodium lamps tends to distort colors with a poor CRI. This type of lighting is adequate for roadway illumination, but it is not ideal for pedestrian-oriented environments. Efficient white lighting sources in the warm white range (2900K-3200K) such as metal halide, induction lighting, and compact fluorescent are best for pedestrian-oriented environments or wherever color is critical (e.g. automobile dealers and high-end shopping malls use warm white lamps to light their outdoor environments). Color accuracy is also important for nighttime safety and policing (see image at right).

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However, in discussing the benefits of warm-white halide lighting, it is also important to note that these sources generally have slightly higher energy consumption, and in some cases, a shorter lamp life than high-pressure sodium. The very long life of induction lamps may help offset their slight energy disadvantage when compared to high-pressure sodium lighting.

Utilitarian pole/luminaire types: The majority of streets in the United States are illuminated with a basic, utilitarian pole/luminaire combination. Street lighting technology has evolved to provide energy-efficient lighting with the lowest possible installation and operating cost in the form of “cobrahead” streetlights and poles equipped with high-pressure sodium lamps (as shown in the image at right).

The shoebox light is another common utilitarian pole/luminaire treatment, often used in parking lots. While shoebox and cobrahead lights are quite efficient, these pole/luminaire combinations generally do not enhance the physical character of a street or help create an attractive, pedestrian-scale environment.

Urban Design considerations: Streetlights can significantly contribute to the design and character of the overall streetscape environment; however, as with a number of other streetscape furnishings such as crosswalks and trash cans, treating streetlights as strictly utilitarian roadway elements is a missed opportunity.

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In many cases, streetlights are one of very few public streetscape investments. Due to their vertical orientation, streetlights are highly visible and can noticeably change the look and feel of a streetscape.

As such, there are several points of urban design to consider:

The size and character of streetlight poles/luminaires should correspond to the importance of the street within the district.

Streetlight spacing should be dictated by engineering, safety, and placemaking considerations.

Streetlights may be architecturally aligned with each other or with focal points along the street, to add to placemaking.

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When both pedestrian and vehicle-appropriate lighting are required in an area, pedestrian-height luminaire heads may be added to roadway height poles (as shown in the image above).

Pedestrian-height luminaires/poles may also be added at more frequent intervals between the roadway height poles.

The architectural style or character of a streetlight should draw from and complement the architectural and historic character of the district, as in the image at right.

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Individual streetlight poles should include elements designed to help create a more human-scaled streetscape and corridor (e.g. a decorative base treatment).